In a High Voltage Direct Current (HVDC) system, a method by which AC power is converted into DC power and then transmitted, and the transmitted DC power is converted into AC power is used, and the related art is disclosed in Korean Patent No. 1019683.
Korean Patent No. 1019683 relates to a voltage-sourced HVDC system and includes a voltage-sourced HVDC apparatus, a sensor unit, and a converter controller.
The voltage-sourced HVDC apparatus converts 3-phase AC voltage into high voltage DC or converts high voltage DC into 3-phase AC voltage through a converter that includes semiconductor switching elements, and the sensor unit detects 3-phase current, apparent power, and active power from the 3-phase AC voltage. The converter controller receives the detected 3-phase current, apparent power, and active power, and generates a D-axis signal and a Q-axis signal. Here, the converter controller includes a D/Q controller and a PWM unit. The D/Q controller receives the D-axis signal and the Q-axis signal and generates an active power D-axis signal and an apparent power Q-axis signal, and the PWM unit generates a pulse-width-modulated signal for turning on or off switching elements based on the output signals from the DQ controller.
The conventional converter controller, such as one disclosed in Korean Patent No. 1019683, namely, an HVDC converter station controller, is operated by being supplied with power from an AC active power grid, which may supply power by being connected to an Alternating Current (AC) transmission network, in order to control the switching elements of the HVDC converter and devices in the converter station. However, the HVDC controller of an independent AC passive power grid, which includes no AC power source but only has loads, cannot be supplied with power from the grid when it is initialized or when it is in a power failure situation, thus it uses a power generator or a battery. For example, in a voltage-sourced HVDC converter station connected to an offshore wind power generation farm, which is a passive power grid, a power generator or a battery that is capable of supplying power to an HVDC controller for quite a long time (about 72 hours) is necessary in order to prepare for a power failure situation or a regular overhaul of the offshore wind power generation farm.
In the conventional HVDC system, if a power generator is used as a power source for the HVDC controller of an independent passive power grid, when it is necessary to resume the operation of the power generator after an interruption due to a power failure or the like, the operation may be resumed through communication in the state in which the grid is located hundreds of kilometers away, such as in the case of an offshore wind power generation farm. However, in this case, an additional battery is required in order to control the power generator. Meanwhile, when a battery is used to supply power to an HVDC controller, because it must supply power for a long time, a high-capacity battery is required.